Missing objects from old Blender in 2.8 (hidden in 2.79b and lost in 2.8)
Ok.. yeah... something new... thx @rjg to pointing into the right direction...

The restriction toggle let's me enable or disable single objects from the viewport - which has nothing to do with the objects visibility! 

Turing the restriction thing on I was able to enable the object in the viewport: 


I find it rather confusing that hidden objects (=visibilty!) in 2.79b become disabled (and NOT hidden) by opening in 2.8.  I would expect that a hidden object in 2.79 will be hidden in 2.8 and not disabled... 

But great - something important learned about the switch from 2.7 to 2.8...